There are a variety of compositions and processes to make precast and cast-in-place lightweight hydraulic articles. Some of these use lightweight fillers and aggregates to achieve lightweight. Some reduce their density by filling the inorganic body with tiny gas bubbles. There are many ways to make lightweight hydraulic articles containing gas bubbles. One of the most common methods is aeration. Aeration can be done mechanically by foaming a freshly prepared cement or gypsum slurry containing a foaming agent and a foam stabilizer. Another way to aerate cement or gypsum is to add a pre-generated organic foam into the slurry.
To generate lightweight cement or gypsum with good texture and mechanical performance by the aeration of conventional cement or gypsum is generally difficult. This is due, in the case of cement, to the bubble collapse in the slow hardening process after hydration. Yet another cause for the collapse of cement and gypsum bubbles is the incompatibility between the organic prefoam component and the inorganic slurry component. This latter collapse usually takes place immediately after the contact of the prefoam with the slurry.